I already wrote how to install Raspbian on Mac, but how to backup it?
Check how your card looks in system..
sudo su - diskutil list
My card is there as /dev/disk2. Now umount the partition boot which is mounted automatically.. Do not forget to change disk number!
diskutil unmountDisk /dev/disk2s1
Go to the folder where you want to have backup and create a backup of SD card (it will take time – depends on your SD card size..).
cd /where_you_backup_will_be dd bs=1m if=/dev/rdisk2 of=raspbian_backup_`date +%Y%m%d`.img
It will create raspbian_backup_20130727.img (for example).
And onliner for h4x0rZ (you need to rewrite the path and diskX
cd /where_you_backup_will_be; diskutil umount /dev/diskXs1; dd bs=1m if=/dev/diskX of=raspbian_backup_`date +%Y%m%d`.img